The present invention relates generally to hydroelectric power generation, and in particular, to a hydroelectric power generating plant which affords improved versatility and applicability in use and construction.
Ideally, a hydroelectric power generating plant is designed in conjunction with an associated dam so that provision is made for penetration of the dam by the penstocks required to conduct water to the turbines, and so that construction of the plant may take place simultaneously with construction of the dam. However, under certain circumstances, the simultaneous design and construction of a dam and power generating plant is not possible. For example, as a result of recent increases in the demand for power, it has become desirable to retrofit many existing dams and spillways, originally constructed primarily for purposes of flood control, water supply, navigation, and recreation to now provide a means for generating electric power. Alternatively, many sites which are capable of being dammed to develop a head which is sufficient to generate electric power are incapable of such use because the cost of constructing the dam and associated power plant by conventional means would not be justified by the expected quantities of power which the resulting station could produce.
Turning first to the retrofitting of a dam, the primary difficulty to be addressed is the manner in which the dam or spillway then in place is to be modified to permit the passage of water through the dam and to the turbines in the power generating station. Traditionally, two methods have been used in this regard. First, portions of the dam have been cut away or opened to develop a desired flow path. Second, penstocks or open channels have been created around the abutments adjacent to the existing dam or spillway. In a few instances, siphon penstocks have been used between fixed intake structures and power generating stations attached to the dam in the conventional manner. In any event, such constructions have generally been avoided due to a variety of practical considerations.
For example, it will be understood that the technique of retrofitting water passages either through or around existing dams gives rise to the potential for certain dangers to person and property which must be heavily weighed when considering the benefits to be potentially derived. Structural modification of the dam or spillway, or excavation of the dam's abutments, requires that painstaking care be taken to avoid the possibility of damaging the installation or creating leakage paths that might result in progressive failure of the dam. Moreover, irrespective of the technique used, it then becomes necessary to temporarily divert or exclude water in the vicinity of those portions of the installation being modified since it is not possible to undertake such modifications in the presence of water. Such exclusion is traditionally accomplished by means of a series of cofferdams surrounding the vicinity of the installation under modification to provide a temporary damming structure capable of holding back the water in that area.
Also to be considered is that modification of an existing installation, irrespective of the method used, as well as the construction of temporary cofferdamming to enable such modifications to be made, is an exceedingly costly operation which can often preclude the economic feasibility of a given project, particularly a project which is only expected to be marginally productive.
Construction costs are also of primary importance in connection with the development of new dam sites at locations which are capable of producing hydroelectric power, but only in marginally acceptable quantities. This would particularly apply to smaller streams, or streams exhibiting relatively poor flow gradient characteristics, in turn decreasing the head which is potentially available for power production and accordingly limiting the amount of power which such a site is capable of producing. Often, it is not feasible to develop an available site only because the quantities of power which can be expected to be derived from the installation do not justify the anticipated costs of design, construction and maintenance.
Another difficulty which has been encountered in connection with new dam constructions, either marginally productive or otherwise, is that desired construction and operational parameters must be balanced against certain environmental considerations necessary in protecting the ecological system then in effect at the site under construction. For example, numerous regulations have been imposed to assure that diverted water flows do not unacceptably compromise the ecology of the dam site, or adversely affect wildlife (aquatic and otherwise) inhabiting the dam site. One such regulation involves the preservation of "in-stream" flow conditions, requiring that water being diverted through the power station at or below the dam actually be selectively released over the dam in specified proportions in the event that a river's original flow characteristics cannot be at least marginally maintained. Such requirements have generally been found to be inconsistent with the requirements of constructing hydroelectric power stations, which are inherently disruptive of in-stream flow in their most economically feasible form. This is because the major components of a hydroelectric power generating station are most conveniently installed at a river bank location on either side of the dam in order to provide convenient and economical access for construction personnel, and to facilitate installation of the relatively heavy equipment which will ultimately comprise the power plant. Such placement necessitates a significant diversion of the river's flow to reach the power generating plant when in operation, leading to a disruption of "in-stream" flow characteristics at the installation site.
A final consideration which cuts across all of the foregoing considerations is one of cost. Several difficulties in achieving cost-effectiveness have already been discussed in relation to the several construction methods described above. Other cost considerations also present themselves, not the least of which is that irrespective of the construction involved, significant portions, if not all of the construction must take place at the dam site. This may present difficulties in that the work site is remote and not readily accessable to construction personnel, or for the delivery of materials to the work site. Also to be considered are problems in servicing the installation after it has been constructed. Obviously, a site which is inconveniently accessed during construction will be inconveniently accessed during subsequent servicing procedures, particularly major overhauls.
Most of these limitations in hydroelectric power plant construction have long been recognized, but have largely been ignored since the number of economically feasible construction sites available were considered sufficient to satisfy then existing power demand. However, it has recently come to pass that this is not the case, and that additional sites must now be developed to satisfy future demand projections without making use of non-renewable sources of energy. This is particularly so in view of the Federal Power Act, which mandates that the Federal Energy Regulatory Commission encourage maximum use of renewable resources such as hydroelectric power in the best interest of the public. Accordingly, a need has recently arisen to develop new devices for producing hydroelectric power at sites which have previously not been economic to develop and which rise to the mandates of present regulations and demand.
One such attempt which is noteworthy in this regard is a "Modular Hydro-Dam" which was developed for the Department of Energy under Cooperative Agreement No. DE-FC07-80ID12205 in July, 1981. The concept involved a modular construction which essentially incorporates all of the features of a dam and a hydroelectric power plant in a single unit capable of being transported over water to the site under construction and then grounded in place upon a suitable foundation formed at the site. This afforded significant versatility in that the only major modifications needed at the installation site related to construction of a foundation, all other major aspects of construction occurring at a shipyard or other appropriate construction site which was more conducive to prefabricated construction techniques at reduced costs. While providing an interesting approach to the problem, such a concept was not generally applicable to the retrofitting of dams and spillways which were already in place, and accordingly was not deemed capable of providing a universal solution to the problems discussed above.
It therefore remained desirable to develop a hydroelectric power plant construction which is adaptable to each of the several different kinds of installation discussed above, and which is capable of meeting present design requirements and environmental regulations.